1. Field of the Invention
The present invention generally relates to automatic control systems for controlling the output power of broadband signal generators and, more particularly, to automatic control systems that compensate for temperature sensitive components in broadband signal generators.
2. State of the Art:
In control systems for controlling the output power of broadband signal generators, such as microwave generators, the performance of various components in the systems can vary with output frequency, output power, and operating temperature. To reduce uncontrolled output power variations from such signal generators, it is known to employ negative feedback loops that incorporate logarithmic amplifiers. In practice, the logarithmic amplifiers usually are PN junctions which, by nature, have logarithmic behavior.
Although logarithmic amplifiers are effective in limiting microwave gain changes in the forward gain path in microwave signal generators, those amplifiers ordinarily do not compensate for temperature-related variations. Accordingly, it is common practice to augment feedback control loops with temperature-compensating thermistor networks.
Usage of temperature-compensating thermistor networks in microwave signal generators is not completely satisfactory, however, because such networks require painstaking determinations of detector temperature drift characteristics across the full range of operating frequencies, temperatures and power levels of the host signal generator. Specifically, temperature drift tests must be performed on a large sample of the temperature-sensitive components, such as microwave detectors, so that components temperature drift characteristics can be accurately determined. In practice, the tests usually are performed over the full temperature and frequency range of the microwave generator (e.g., 2 to 26 gigahertz, and 0 to 55.degree. C.) and at different power levels (e.g.,-10 to +10 dbm). Such testing not only consumes time but is expensive.
In view of the preceding discussion, it can be appreciated that a need exists for improved automatic power control systems that compensate for the performance of temperature-variable components in broadband signal generators.